1. Field of the Invention
The present invention relates to a laser recording apparatus, in particular, to an apparatus including a semiconductor laser as a light source and permitting half tone recording, such as a digital copy machine or a laser printer.
2. Description of the Related Art
These days, a laser printer incorporated with electrophotographic technology and laser scanning technique has rapidly spread as a computer output device or a digital copy machine, since in such an apparatus, ordinary paper can be used and an image of high quality can be obtained at high speed.
A conventional laser scanning optical system is shown in FIG. 1. A laser beam 2 emitted from a semiconductor laser 1 and modulated according to image signals is reflected through a lens 3 on a surface of a rotating polygon mirror 4, thereby to form an image as a micro spot on a photosensitive body 6 through an image forming lens 5. This micro spot is two-dimensionally scan exposed on the photosensitive body 6 with the rotations of the polygon mirror 4 and the photosensitive body 6 to form an electrostatic latent image. A light-intercepting element 7 for controlling the start position of image writing in a main scanning direction is placed outside the range of an image at the scanning start position side thereof on a scanning line.
To realize an optical system outputting copies of a picture of A4-size at the rate of 100 sheets/min in the laser printer as above, the peripheral velocity of the photosensitive body 6 should be 500 mm/sec and the revolution number R(rpm) of the polygon mirror 4 is computed in the following formula; EQU R=Vo.times.DPI.times.60/(25.4.times.n)
where Vo is the rotating velocity of the photosensitive body 6, DPI is the number of recorded dots/inch, generally between 300 and 400, and n is the number of reflecting surfaces, generally between 6 and 10. Substituting now Vo, DPI and n for 500, 300 and 8 respectively in the above formula, the revolution number R of the polygon mirror 4 comes to 44,291.
In reality, however, for the polygon mirror with such a revolution number, conventional ball bearings cannot be used as a bearing for supporting a rotation axis, and consequently specific bearings such as fluid bearings and magnetic bearings are required, which causes a cost increase. In addition, since the modulation frequency of the laser 1 as a light source becomes higher, accelerated data transferring from a laser control circuit and a host machine is needed, which also causes a cost increase.
There is an alternative method of accelerating data transfer in which laser beams from a plurality of light sources are deflection scanned on the surfaces of a rotating polygon mirror to make simultaneous recordings for a plurality of lines. Scanning with a plurality of laser beams, when the number of laser beams is M, reduces the revolution number of the above polygon mirror and the laser modulation frequency to 1/M, which results in a steep cost-down.
As an example, a recording apparatus is disclosed in the Japanese Patent Application Laying Open (KOKAI) No. 59-112763. The apparatus having a semiconductor laser array, as a light source, consisting of a plurality of semiconductor lasers includes an optical system for forming images on a recording body of adjacent points of the cross-sectional forms of emitting beams from respective semiconductor lasers and a driving circuit for independently driving respective semiconductor lasers, thereby to enable a batch scanning of laser beams from a plurality of semiconductor lasers.
However, in a conventional optical system with one laser beam, the beam comes onto a rotating polygon mirror vertically in a sub-scanning direction with respect to the reflecting surface of the mirror, while in an optical system scanning a plurality of beams on one and the same rotating polygon mirror, the laser beams are emitted onto the reflecting surface of the mirror at a slight angle. Consequently, on the scanning surface (a recording body) a plurality of scanning lines become curved in the sub-scanning direction as illustrated in FIG. 2. The curvature of a scanning line increases, as an incident angle of the beam on the polygon mirror with respect to the scanning surface thereof is shifted farther from the vertical, that is, a beam nearer either side of the beam group simultaneously scanned (as a beam is shifted farther from an optical axis of a lens diameter) has bigger curvature in the scanning line thereof.
Therefore, when a recording is made in such an optical system, a pitch of a scanning line fluctuates at every period consisting of the number of light sources (the number of semiconductor lasers). These pitch fluctuations are actualized as unevenness of exposure on a recording medium and as unevenness of density on an image.
Such unevenness of density is not so noticeable in binary images such as a letter, however, when an exposure pattern is varied (varying the number of dots, the duration of lighting a light source laser and the emission power of a laser) according to the shade of an image within a dot matrix consisting of a plurality of micro pixels (dots) to make half-tone recording, unevenness of density called banding occurs, which causes gross deterioration of image quality.